Test of electron delocalization effects on water-proton spin-lattice relaxation by bromination of [tetrakis(4-sulfonatopheny)porphine]manganese

The potential value of electron spin delocalization as a means for substant ially increasing the ability of a paramagnetic metal complex to induce nucl ear spin relaxation of water protons has been examined by covalent attachme nt of bromine atoms in the beta-pyrrole positions of the [5,10, 15, 20-tetr akis(4-sulfonatophenyl)-21H,23H-porphine]manganese complexes[(MnTPPS)-T-III ](3-) and [(MnTPPS)-T-II].(4-) The water-proton spin-lattice relaxivities a re reported as a function of magnetic field strength for the brominated and nonbrominated metalloporphyrins over the range of magnetic field strengths corresponding to proton Larmor frequencies between 0.01 and 30 MHz. The br ominated metalloporphyrins increase the water-proton relaxativities compare d to the nonbrominated metalloporphrins, and, at low magnetic field strengt hs, the brominated [(MnTPPS)-T-II](4-) complex rivals the efficiency of the hexaaquomanganese(II) ion. Attempts to fit the experimental data to theori es for paramagnetic relaxation, which are based on the point-dipole approxi mation, result in distances between the paramagnetic center and the water p roton that are unreasonably short based on published structural data. The e xcess relaxivity implies that the point-dipole approximation may be inappro priate for these porphyrin systems and electron spin delocalization may pro vide a significant contribution to nuclear spin relaxation that may be frui tfully exploited in construction of contrast agents for magnetic resonance imaging.